BROOD PARASITISM, NEST PREDATION, AND SEASON-LONG REPRODUCTIVE SUCCESS OF A TROPICAL ISLAND ENDEMIC

The Puerto Rican Vireo (Vireo latimeri) is a single-island endemic res ident on Puerto Rico. The Shiny Cowbird (Molothrus bonariensis), a gen eralist brood parasite native to South America, arrived on the island in 1955 and has established itself as a breeding resident. To determin e the impact of the exotic cowbird on vireo reproductive success, I st udied the demography of marked Puerto Rican Vireos in Guanica Forest, Puerto Rico, in 1990-1993. Vireo breeding season length varied among y ears (69-106 days), apparently influenced by rainfall. The primary cau ses of reproductive failure were nest parasitism and nest predation. C owbirds parasitized 73-83% of vireo nests. Parasitism reduced the numb er of vireos hedged per successful nest by 82%, primarily through decr eased hatching success and hedging success. Vireos did not abandon nes ts in response to cowbird egg-laying, but frequently deserted if cowbi rds removed host eggs. Native avian predators and exotic mammalian pre dators together caused the demise of about 70% of all nest attempts. A s a result, daily nest survival rate was low (0.93 +/- 0.01), and only 13-19% of nests fledged vireo or cowbird young. Pairs renested after failure and attempted to raise second broods. Females in this populati on attempted two to six nests per season and fledged an average of 1.3 3 vireos in 1991 and 0.24 vireos in 1993. The combination of restricte d breeding season, high predation and parasitism rates, large impact o f parasitism on reproductive output, and low seasonal fecundity of fem ales suggests that, despite high survival rates, the Puerto Rican Vire o is in danger of extirpation from portions of its range.